The present invention relates to a mass driver circuit for weaving looms, and more particularly relates to improvement in construction of a driver circuit for accurate inching operation on weaving looms.
When a trouble such as yarn breakage of unsuccessful weft insertion occurs during normal running of a loom, the loom is either manually or automatically stopped and inching operation is performed in order to move the crank to a position suited for work to remove the cause of the trouble and/or weaving defects resulted from such a trouble.
One conventional electric circuit for such inching operation includes a push button switch which is given in the form of a manually operable, self-returnable contact so that electric power should be supplied to the drive motor for the loom as long as the push button switch is turned on. That is, the loom is driven for inching operation by turning on the push button switch for prescribed length of period so that the crank should be moved to a desired position.
Alternating current motors such as induction or synchronous motors are in general used for drive of looms, and such a drive motor is connected, for rotation, to an electric power source which delivers electric power of commercial frequencies. Such an electric power source is called "a commercial frequency power source" and will hereinafter be described as "a CF power source". So, once the CF power source to be used is fixed, the rotation speed of the drive motor per se is unchangeable. In order to obtain a desired running speed of the loom for which the drive motor is used, transmission ratio has to be changed in the power transmission coupling the drive motor to the crank shaft of the loom. More specifically, diameter of a pulley or pulleys used in the power transmission has to be changed.
With recent significant rise in running speed of looms, the diameter of pulleys for the power transmission has been increased accordingly. When the drive motor is connected to the CF power source also at inching operation on the loom under this condition, even short period turning-on of the push button switch results in relatively large rotation of the crank shaft due to the increased diameter of the pulleys, and the crank is moved past the desired position. In addition, since the push button switch is operated manually, turning-on period of the push button cannot be made shorter than a certain limit. So, it is now quite difficult to enable accurate inching operation on looms as long as manually operated push button switches are used.
It was proposed to use a pole-change type motor for drive of looms. In this case, the number of poles in the drive motor is increased at inching operation from that at normal running of the loom. This results in lower rotation speed of the drive motor and the loom is provisionally driven for slower running at inching operation.
Such a pole-change type drive motor, however, requires corresponding increase in number of coil windings which is inevitably accompanied with low efficiency. In addition, reduction in number of coil windings at normal running causes lowering motor efficiency. In order to cover the deficit, it is necessary to use a drive motor which can generate torque large enough to enable proper normal running of the loom. This inevitably connects to large construction of the loom. Further, since this system relies on change in number of poles in the drive motor, it is difficult to change rotation speed of the loom over a significantly wide range. So, in particular under high speed running condition, the rotation speed of the drive motor cannot be lowered enough at inching operation. In addition, change in rotation speed of the drive motor can be effected stepwise only, and, as a consequence, rotation speed of the drive motor cannot be adjusted continuously. So, it is difficult to move the crank of the loom always to a correct position at inching operation.
In most weaving factories, looms are divided into several groups depending on, for example, the type of product to be woven and a mass drive system is employed for each group. Under normal condition, only one or two looms in a group require concurrent inching operation. Despite this real condition, the above-described pole-change system requires that every loom should be provided with a switch circuit pole change. This apparently causes rise in the installation cost.